1. Field of the Invention
Exemplary embodiments of the present invention relate to a voltage-controlled oscillator robust against power noise and a communication apparatus using the same; and, more particularly, to a voltage-controlled oscillator robust against power noise, which is configured to detect noise introduced through the power supply in advance and remove the power noise through a negative feedback loop so that a stable, single oscillation frequency is provided while guaranteeing low phase noise and low jitter characteristics, and a communication apparatus using the same.
2. Description of Related Art
As generally known in the art, transceiver circuits for communication, which are manufactured as chips, tend to be adversely affected by electromagnetic interference (EMI) introduced through the power line from spurious external electromagnetic environments, or by change of temperature inside/outside the chips.
Specifically, EMI noise induced by change in the power supply voltage may have a critical influence on the voltage-controlled oscillator (VCO) within the IC, which governs the system's communication performance. This is because the frequency quality of the VCO determines the signal modulation/demodulation performance of the mixer for frequency conversion, and any increase in the phase noise or jitter of the oscillation signal is followed by increase in the error vector magnitude (EVM) or bit error rate (BER) of the entire system, eventually degrading communication performance.
As used herein, the VCO refers to a circuit configured to vary the frequency based on tuning voltage, and generally has a phase-locked loop (PLL), as illustrated in FIG. 1, to maintain a stable frequency. FIG. 1A illustrates the construction of an exemplary conventional radio receiver, and FIGS. 1B and 1C illustrate exemplary frequency characteristics of the radio receiver illustrated in FIG. 1A.
Referring to FIG. 1A, the frequency synthesizer 100 is a circuit configured to control the frequency of the VCO 110 by means of the PLL.
The VCO 110 generates an oscillation frequency fvco, which goes through a divider 120 to become fvco/N, and which is compared with a reference frequency fref by a phase/frequency detector (PFD) 130. The reference frequency fref is generated by a temperature-compensated crystal oscillator (TCXO). The PFD 130 generates a control signal corresponding to the phase/frequency difference between two frequencies to adjust the amount of charge of the charge pump 140. The charge pump 140 then sends a corresponding displacement current to the filter 150 to increase/decrease the tuning voltage Vc as much as the phase/frequency different between both frequencies. The frequency of the VCO 110 is adjusted in this manner.
The above-mentioned PLL is repeated to adjust the frequency until the VCO 110 stabilizes the tuning voltage Vc. In other words, the VCO 110 adjusts the frequency so that the reference frequency fref and the multiplied arbitrary frequency fvco/N become finally identical or similar. As such, the VCO 110 fixes the center frequency through the PLL. Due to characteristics of the PLL, noise is filtered through a high-pass filter about the center frequency.
However, the above-mentioned conventional VCO 110 has a problem in that, if EMI-induced noise enters its power supply from the outside, the PLL of the VCO 110 becomes less stable, as illustrated in FIG. 1B, increasing the phase noise or jitter of the oscillation frequency. Therefore, there is a dire need for a VCO which, even if power noise is introduced from the outside, can provide a stable oscillation frequency with low phase noise and low jitter characteristics, as illustrated in FIG. 1C.